Variable displacement axial piston pumps and motors have long been used in industry. The basic axial piston pump and motor includes a rotatable cylinder barrel containing several pistons which reciprocate in mating piston bores more or less parallel to the axis of a drive shaft. One end of each piston is held against a tiltable swashplate. When the swashplate is tilted relative to the drive shaft axis, the pistons reciprocate within their bores and a pumping action occurs. Each piston bore is subjected to two main pressure levels during each revolution of the cylinder barrel. One pressure is a result of the load and is located on one side of the ramp of the tilted swashplate. The other pressure is normally much lower and is located on the other side of the swashplate ramp. As the piston bores sweep past the top and bottom dead center positions, torque moments are generated on the swashplate as a result of the reciprocating pistons and pressure carryover within the piston bores. Pressure carryover is the time delay in pressure rise in the piston bore as the piston bore is going from low to high pressure or the time delay for pressure decay when the piston bore is moving from high to low pressure.
The swashplate is typically controlled using one or more actuators and a bias spring to offset the torque moments. The torque moments are quite high in today's high pressure axial piston units such that the actuators are quite large and may account for approximately 20% of the overall size of the pump or motor. Swashplate response and control response are limited because of the volumes of fluid that need to flow into and out of the hydraulic actuators and the total added inertia of the actuators. Moreover, such actuator system within the pump contributes from about 7-12% of the overall cost of the pump. These costs result from the number of pieces used in the actuators and the precision machining of several large pieces and the expense associated with assembly of the pump or motor.
There have been at least two proposals to control the angle of the swashplate by using the pistons within the cylinder barrel instead of a separate actuation system. One such unit is disclosed in Japanese Utility Model Application No. 61-37882. Another unit is disclosed in U.S. Pat. No. 4,918,918. One of the disadvantages of those disclosures is that the swashplates are controlled hydromechanically. It is believed that at least one operating parameter should be sensed electronically and the output signal processed electronically for adjusting the position of the swashplate of today's high speed units. For example, many of today's pumps rotate at about 2,250 revolutions per minute, which calculates to be about 37.5 revolutions per second. If such pump has 9 pistons, a total of 338 piston bores sweep past each dead center position each second. This means that about 0.003 seconds elapses between consecutive piston bores and the control system has somewhat less than 0.003 seconds to adjust the pressure rise/decay of each piston bore.
Thus, it would be desirable to provide a variable displacement axial piston hydraulic unit with the capability of changing the displacement of the swashplate by modulating the pressure in the piston bores at top and bottom dead center positions of the pistons to thereby modify the force imposed on the pistons as they pass through the top and bottom dead center positions for controlling the swashplate position wherein modulating the pressure is controlled electronically based on at least one operating parameter of the unit.